1. Field of the Invention
This invention concerns sorption analysis and, more specifically, a method for compensating for the time-dependent change in the coolant level surrounding the sample cell during gas sorption analysis.
The measurement of the amount of gas adsorbed and desorbed by a solid as a function of gas pressure is employed in the determination of surface area, pore size, pore volume and pore area distribution of many solid substances. These substances can be non-porous or porous to a greater or lesser extent and can be in the form of monoliths, granules, pellets, tablets, extrudates, powders or other solid form. The surface area of a solid is an important physical characteristic which plays a significant role in the behavior of the solid in terms of its interaction with other solid surfaces, liquids, vapors and gases. Pore size, volume and area distributions are important in catalyst selectivity, molecular sieving action, gas and liquid absorption capacity, optical properties of transparent materials and bio-compatibility of implanted materials.
The method most commonly employed to make these surface area and other determinations is the so-called static volumetric method and requires that the amount of gas adsorbed be measured as a function of applied gas pressure. In this method, the sample is contained in a sample cell normally constructed of borosilicate glass. Suitable pretreatment of the sample removes unwanted surface contaminants. The sample cell containing the sample then is attached to the measuring apparatus and evacuated to remove much of the residual atmosphere or other gas. A valve separates the test station, to which the sample cell is attached, from the remainder of the system. The void volume is that volume contained within the sample cell and the test station up to that valve and must be known in order to calculate the amount of gas in that void volume. This void volume determination can be achieved in one of at least two ways. In the classical technique, the void volume is measured immediately prior to the sorption analysis. A non-adsorbing gas, such as helium, is expanded or dosed from a known, calibrated volume, the dosing manifold, into the sample cell containing the sample. The principles of the gas laws, primarily Boyle""s Law, are applied. This determination must be done both at ambient temperature and then with that portion of the cell containing the sample at the analysis temperature. Alternatively, and in the preferred embodiment of the invention, the void volume is not calculated directly, but the amount of analysis gas, the adsorptive that is transferred from the manifold to an empty cell, is measured as a function of pressure, under the same conditions of temperature as the subsequent analysis, and can be done at one or more discrete pressures.
The analysis temperature normally is no higher than the boiling point of the adsorptive gas. Since the analysis gases most commonly employed are inert, xe2x80x9cpermanentxe2x80x9d gases, the analysis temperature should be that of the liquefied gas. This enhances the adsorption process; therefore, a cryogenic liquid is employed. For example, when nitrogen is used as the adsorptive, the cryogenic liquid most commonly is liquid nitrogen and is held in a Dewar flask into which the sample cell is immersed. After the void volume has been determined in the manner described above, the proportion which is effectively at ambient temperature, the so-called xe2x80x9cwarm zonexe2x80x9d, and that which is effectively at the cryogenic temperature, the so-called xe2x80x9ccold-zonexe2x80x9d, must remain constant. Only then can the amount of gas in the total void volume be accurately accounted for.
During an adsorption analysis, for an accurate determination to be made, pressure in the sample cell must increase solely due to the addition of gas from the dosing manifold and not due to the warming of any portion of the void volume. However, during the measurement, the coolant evaporates from the Dewar. Thus, the cold zone decreases in volume, while the warm zone increases. If this change of coolant level is not compensated for by some means, the measurement of the amount of gas adsorbed is in error, which is a function of both pressure and time.
At low pressures, there exists less gas in the void volume than at higher pressures. Therefore, any change in temperature at low pressure imparts a smaller error than an identical change at a higher pressure. Since the affected volume in creases with time, it is essential to make the compensation also a function of time. During a complete adsorption/desorption isotherm analysis, the pressure in the cell increases during adsorption, then decreases during desorption, due to the normal measurement process. In this case, there exist two data points for each pressure value, one adsorption and one desorption. However, even though the pressure in the cell might be identical, the data points never can be recorded at the same instant in time and therefore the degree of compensation or i.e. the amount by which the sorption data is to be corrected, is not identical.
2. State of the Prior Art
Control or maintenance of cryogenic liquid level, the coolant, which surrounds the sample cell, which is changing because of its continuing evaporation, can be achieved in a number of ways:
a) The coolant can be replenished during the analysis by transferring coolant from a storage vessel to the Dewar, in response to the output of an electronic circuit monitoring coolant level, as employed in Micromeritics(copyright) Digisorb 2600, Coulter(copyright) Omnisorp(copyright) 100/360 and Carlo Erba Sorptomatic.
b) The analysis Dewar can be raised by a motorized elevator in response to the output of an electronic circuit monitoring the coolant level, as in the Quantachrome Autosorb.
c) A porous tube, placed around the stem of the sample cell, draws coolant up to the top of that tube by capillary or wicking action, regardless of coolant level around the porous tube, as in several products of Micromeritics.
The changes in the amounts of gas in the cold and warm zones of the void volume, due to the uncontrolled evaporation of coolant, can be minimized in a number of ways, for example d), e) and f), next described; which can be used alone or with any of a) through c):
d) The stem portion of the sample cell is made as narrow as reasonably practical. This reduces the actual volume affected by coolant level change.
e) The void volume can be reduced further by the use of filler-rods, as taught by U.S. Pat. No. 5,360,743, Lowell. Extreme amounts of said volume filling can be achieved by the use of rods manufactured from polytetra-fluoroethylene (PTFE), as in the Quantachrome(copyright) NOVA(copyright) analyzer, and can be machined to provide a tighter fit than can rods made of glass.
f) The stem portion of the cell can be insulated from the coolant by means of a tight-fitting jacket of material with poor thermal conductivity, or by containment within an evacuated or partially evacuated vessel, which can be silvered as in the manner of a Dewar flask.
The problems with the prior art control means are that a) through c) require some physical contrivance to actually control the coolant level, which can be less than successful and entails added costs, risk of mechanical and/or electrical failure and normally sets limitations as to the type and size of sample cell and/or Dewar. The narrow stems of d) and e) set limitations as to the size of sample which can be admitted into the sample cell. Also, tight fitting filler rods can limit evacuation rates of the sample cell; evacuation being a prerequisite for this type of volumetric analysis. Insulation means f) is not particularly effective, since it is exactly this type of construction in the surrounding Dewar which is unable to maintain the coolant level, in the first place. Furthermore, this prior art adds significant extra cost to the fabrication of the sample cell and increases fragility.
This invention provides method usable with sorption analyzers for temperature compensation due to coolant level changes around the sample cell. Coolant level changes are due to progressive evaporation of the coolant in the Dewar and cause relative temperature changes in cold and warm zones of the void volume of the analyzer. Such temperature changes generate time-dependent errors in the sorption analysis. This invention, throughout the duration of a gas sorption analysis, in xe2x80x9creal timexe2x80x9d, provides temperature compensation, without the need for any physical or mechanical contrivances. Thus, this invention enables correction of the error, otherwise introduced due to the changes in cold and warm zones, in the determination of volumes of gas adsorbed and desorbed. This invention permits the coolant to evaporate and, in xe2x80x9creal timexe2x80x9d, provides for mathematic corrections to the sorption data and the output from the analyzer; a novel solution to the problem.
The determination of the change in the volume of the cold zone and the amount of gas in the ever increasing warm zone requires the use of known as well as easily ascertained data, determination of other and variable information and the use thereof in formulas, which themselves are known to those skilled in the art, but are employed in a novel manner. One piece of predetermined information is (xcex94H), the changing level of the evaporating coolant, which can be predetermined experimentally by the manufacturer or by the analyzer user and stored, as in a lookup table, in the analyzer program; can be plotted graphically and then curve fitted by equation. See equation (A) further below. The changing coolant level (xcex94H) also is employed in a known formula (B) set forth below to obtain the value for the decrease in adsorptive gas in the cold zone (xcex94V cold zone) over a given time period. The change of that affected portion of the cold zone temperature also is a needed piece(s) of data.